I remember flying in the 1980s -- seats had 4 inches of cushion and legroom was great. (Also, you could be picked up or dropped off not just at the airport, but at the gate -- this is extraordinarily convenient for novice travelers, children, and so forth).

This article points out that passengers have waist sizes that are 2-inches larger than they were 20 years ago -- the timeframe over which seats and legroom have been getting smaller.

Spirit is the worst in this regard.